Munitions carrying missiles achieve flight and are propelled at high velocities by the thrust of expanding gases created by a hypergolic reaction between a fuel and an oxidizer, the propellants, released from reservoirs or storage tanks, as variously termed, carried by the missile. Hence, the propellant storage tanks form a necessary component of a weapon carrying missile, more particularly, of the missile's propulsion system. Propellant storage tanks for the foregoing application are intended for a single use and, hence, are expendable in application. The tank for storing the fuel, such as Hydrazine, N.sub.2 H.sub.4, is substantially the same tank structure to store the accompanying Oxidizer, N.sub.2 O.sub.4, with which the fuel reacts both of which are known propellants. The effectiveness of the missile depends not only on the quantity of propellant carried, which is related in part to the missiles intended target range, but also upon the means by which the propellant is effectively extracted from the storeage reservoir and efficiently consumed. The storage tank's cost of manufacture, volume efficiency and weight are important considerations in tank design.
Propellant storage tanks for the foregoing missile application must not only withstand high internal pressures with limited radial and axial expansion, but must be light in weight. Typically the propellant tank is formed of a relatively thin thickness of light weight metal, such as Aluminum. To provide greater strength an outer wrap of graphite composite material, which is also very light in weight, is applied to the outside surface of the tank. In forming the wrap, graphite strip material is wound around the outer surface of the tank on all sides. Typically that wrap is formed in two layers; the first is known as a helical wrap and the second wrap, overlying the helical wrap, is known as the hoop wrap. The use of such composite wrap dictates the tank's shape: a cylindrical central section having relatively rounded or dome shaped front and back ends, more specifically ends that are sections of an oblate spheroid. The tank cannot be of a simple "oil drum" shape with flat ends.
In a typical propulsion system of the foregoing type, a gas, stored under high pressure in a separate reservoir, provides the necessary force to force the propellants, whether liquid or gel, out of the respective storage tanks. During operation, when that gas is applied to the propellant tank's inlet, located in the center of one of the dome shaped ends of the tank, the gas pressure is applied to the back side of an internal dome shaped piston. That pressure releases the piston and forces the piston to move axially within the tank, forcing liquid propellant, confined in the tank to the front of the piston, to flow through the propellant tank's outlet, located in another dome shaped end at the other end of the tank, and, thence, through the propellant lines to the missile's engine.
The piston is rigid, convexly shaped so as to conform to the dome shaped wall at the outlet end of the tank, is of uniform thickness and is of a radius that fits within the cylindrical portion of the tank. With dome shaped tank inlet and outlet ends and a cylinder shaped section in between, if one visualizes progressing within the tank from the inlet end to the right along the axis of the tank, while remaining within the inlet dome, the radial distance, or radius from the axis of the tank to the tank side wall, gradually increases until one reaches the base of the dome, at which point the radius attains that of the cylindrical section. Thereafter the radius remains constant until one reaches the dome at the outlet end of the tank. As one progresses further and into the outlet dome, the radius again decreases. Hence in those prior propellant tanks, the piston cannot be initially positioned within the dome shaped inlet portion of the tank, where the radius is smaller than that of the piston, since the piston will not fit.
As a consequence, the cavity within the dome portion at the inlet side of the tank, to the left side of the piston, is unused; it is essentially wasted space that cannot be used to store propellant. As a further consequence the volume efficiency of the tank, the ratio of volume holding propellant to the volume of the tank as a whole, is not as great as desired. With the present invention a portion of the dome section is used to store propellant. Hence, propellant tanks of a given size constructed according to the present invention can carry more propellant than before and possesses greater volume efficiency, a decided advantage.
Prior propellant tanks of this type include two cylindrical shaped metal membranes or diaphragms, as variously termed, an outer one and an inner one of smaller diameter, essentially sleeve like in shape, with one located within the other along a common axis to define an annular region therebetween; and an end of each diaphragm was welded to the aforedescribed piston to form an annular end wall. The propellant is confined partially within that annular shaped region. The relationship between those diaphragms and the rigid piston was such that the axial movement of the piston pulled along the end of each diaphragm and forced each diaphragm to smoothly invert or roll over on itself in great part, a feature called rolling, much like the action that occurs when turning ones stocking inside out and pulling one's bedsheet down, effectively reducing the size of the annular cavity as the piston moves. That rolling action progresses until the piston contacts the inner dome shaped wall at the outlet end of the tank. The present invention incorporates the principles of rolling diaphragms.
Additionally in the foregoing prior propellant tank structure, the outside surface of the outer diaphragm is bonded to an overlying cylindrical metal wall, a liner, which provides structural support and a protective barrier for the more fragile outer diaphragm. That bond was accomplished with a releasible bonding agent, such as one containing Teflon particles. The bonding agent served to preclude the formation of any cracks or pockets between the liner and the underlying diaphragm, since any pressurized gas entering a pocket between the two members could distort or collapse the underlying diaphragm and thereby preclude proper operation. The pulling force created by the piston on the end of the outer diaphragm during piston movement essentially peels the diaphragm away from the overlying liner and allows the diaphragm to be smoothly rolled over on itself without cracking or kinking. Hence the foregoing propellant tank structure is referred to as a bonded rolling diaphragm tank. The foregoing bonded rolling diaphragm tank structure has achieved a degree of success and has been used successfully for over twenty years.
The present invention incorporates the structure of the bonded rolling diaphragms, the overlying metal cylinder overlying and protecting the outer diaphragm and the releasible bond between the outer diaphragm and the protective cylinder by simple modification to the geometry of and relationship of those elements to achieve a propellant tank of greater volume efficiency.
Accordingly, an object of the invention is to increase the volume efficiency of a missile propellant tank.
A further object of the invention is to employ bonded rolling diaphragms in a propellant tank of greater volume efficiency than heretofore available.
And an ancillary object of the invention is to reduce the number of separate pieces required to form a rolling diaphragm propellant tank.